What is a protocol?

Definition

A protocol is a system of rules that defines how data is exchanged inside a computer or between computers. Devices need a shared understanding of the format and meaning of the information they exchange; protocols establish that agreement.

Three elements

Syntax

Syntax defines the structure or format of data. For example, a hypothetical bit sequence such as 01001000101110101 could reserve one section for an address and another for data. The syntax tells us how to separate those fields.

Semantics

Semantics defines what the fields mean and how participants act on them, including rules related to errors, synchronization, and flow control. In the original analogy, 0100 might mean “fourth floor,” while 1000 might mean “water.” The bit pattern is only useful when both parties agree on its meaning.

Timing

Timing concerns what is sent when, and at what rate.

Protocol functions

Different protocols provide different combinations of functions. The original overview lists ten:

1. Addressing. To send data to another endpoint, the sender needs an address appropriate to the layer involved. Protocols define how those addresses are represented and used.

2. Sequence control. Sequence information helps organize protocol data units, or PDUs, and allows a receiver to reconstruct the intended order. TCP is one example that provides ordering.

3. Fragmentation and reassembly. Larger units may need to be divided into smaller pieces for transmission, then reconstructed at the receiving side. The details depend on the protocol and layer.

4. Encapsulation. A service data unit, or SDU, is combined with protocol control information, or PCI, to form a PDU. The SDU carries the information supplied to the layer; the PCI contains control fields such as source and destination addresses or sequence numbers.

5. Connection control. A connection-oriented service establishes and manages a connection for exchanging data. The original contrasts TCP's connection-oriented operation with UDP's connectionless operation.

6. Flow control. Control over the amount and rate of transmitted data helps prevent a faster sender from overwhelming a receiver.

7. Error control. Error-detection and related mechanisms identify problems in transmitted information or control fields. The original examples include parity bits and a CRC, or cyclic redundancy check.

8. Synchronization. Participants coordinate their communication state and agree on relevant parameters.

9. Multiplexing. Multiple communications can share a communication facility or link.

10. Transmission services. A protocol or service may define priorities and handle requirements such as security.

Editorial corrections: the original swapped sender and receiver in its fragmentation/reassembly explanation and used “Protocol Control Unit” for PCI. Those are corrected above. Sequencing is not exclusive to connection-oriented protocols, and these ten functions are a survey—not features guaranteed by every protocol.